I want to get the available options/items of a layered navigation, but I cannot find out how to achieve this.

Basically, I want to know what filters with items are available, when I set some filters, without having a product collection. For example:
I send an request saying size L categories Tshirts
and get returned the colors red and blue are availble, but not the color black.

I just cannot find, where the options of my attributes are being filtered.

What I didn't know was how to filter then the returned items. This actually is done by the block itself.
So you can just send an reqeust like ?attributecode=value to your controller that creates the block and it returns now only the filtered items.